A method of manufacturing a steel pipe product welded by continuously supplying a hot rolled steel sheet and causing both edges thereof to butt against each other while forming the hot rolled steel sheet into a cylindrical shape, is one of conventional methods of manufacturing welded steel pipes. As a method of this welding, electric resistance welding (ERW) is often used, but in recent years, laser beam welding of welding by use of a laser beam as a concentrated heat source has started to be used. In particular, in recent years, high performance laser beam welding machines have been developed, which use semiconductor excited laser, fiber laser, and the like having an oscillation wavelength shorter than that of conventional gas laser, such as CO2 laser, and in which reduction in efficiency caused by generation of plasma due to interaction between the laser beam and the metal to be welded that has evaporated upon welding is hard to occur.
When this laser beam welding is applied to a steel pipe, since melting over a thickness direction of a steel material is required, a penetrated welding state is required to be maintained. However, in a continuous forming process of a steel strip, according to conditions of the manufacturing line, heat input conditions, and the like, the butted position thereof tends to fluctuate. Therefore, there is a need to continuously detect an irradiation point of a laser beam in a laser beam welded portion and to accurately control the irradiation point of the laser beam.
In a method of monitoring welding of this laser beam welded portion, for example, as described in Patent Literature 1, the laser beam welded portion is directly monitored by a television camera, and a weld line (seam position) and a molten pool center position are detected. In this method, the laser beam welded portion is illuminated with external illumination, and on the premise of the weld line being dark and the molten pool being bright as a result of observation thereof, a position of one line for each of weld line detection and molten pool detection is determined in a horizontal direction (a direction perpendicular to a travel direction of the welded steel pipe) on a captured image, and a luminance pattern of that horizontal line is binarized, to thereby detect the positions of the weld line and the molten pool.
Further, according to Patent Literature 2, reflected light, which has been emitted from a light onto a welded portion near a laser beam irradiation portion and reflected from a steel sheet, and plasma light are imaged by an imaging device, and from the captured image, a portion having luminance higher than that of the surrounding steel sheet portion is detected as a laser beam irradiation point.